Subject: Re: mtpr/mfpr TBDATA and TBCHK instructions
To: Toru Nishimura <>
From: Anders Magnusson <>
List: port-vax
Date: 04/03/2000 22:31:08
> Hello, VAX foks,
> Could someone out there teach me about what's the definition and usage
> of mtpr/mfpr TBDATA and TBCHK instructions?
TBCHK: Writing an address to the TBCHK register will set the V flag in
the PSL if the TLB holds a valid translation for that address.
The VAX ARM also says that the TBCHK register is reserved for Digital use.

For TBDATA the VAX ARM says:
"This internal register is used to read and write locations in the TB. On
a MFPR to this register, the page table entry for the virtual address in
P0BR is read from the TB into the destination. On a MTPR, the source 
operand is written into the TB as the page table entry for the virtual
address in P0BR. the results of an MTPR/MFPR on the register are 
UNPREDICTABLE if memory management is enabled."

So, try to understand what the TBDATA is good for :-)

Note that TBDATA only exists on some very old machines.

Why are you asking about this? :-)

-- Ragge